The invention relates in general to radio frequency filter. In particular it relates to filter structures comprising piezoelectric resonators, typically thin film bulk acoustic wave (BAW) or surface acoustic wave (SAW) resonators.
The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. The development leads to increasing requirements on the miniaturization of the components and structures used in the mobile communication means. This development concerns radio frequency (RF) filter structures as well, which despite the increasing miniaturization should be able to withstand considerable power levels, have very steep passband edges, and low losses.
The RF filters used in prior art mobile phones are usually discrete surface acoustic wave (SAW) or ceramic filters. Surface acoustic wave (SAW) resonators typically have a structure similar to that shown in FIG. 1. Surface acoustic resonators utilize surface acoustic vibration modes of a solid surface, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. A SAW resonator typically comprises a piezoelectric layer 100, and two electrodes 122, 124. These electrodes form an Interdigital Transducer (IDT). The shape of the electrodes 122, 124 typically resembles letter E or a comb, and the electrodes are placed so that the fingers of a first electrode are parallel with the fingers of a second electrode and between them. The frequency of a SAW resonator depends most on the distance between the fingers and also on the width of the fingers. Impedance of a SAW resonator depends mostly on the number of fingers and on the length of the fingers. In addition to the IDT, a SAW resonator has typically two reflectors, one on each side of the IDT, for reflecting back the surface acoustic wave induced by the IDT and traversing in a direction normal to the direction of the fingers of the IDT.
Various resonator structures such as filters are produced with SAW resonators. A SAW resonator has the advantage of having a very small size, but unfortunately a SAW resonator cannot withstand high power levels.
It is known to construct thin film bulk acoustic wave resonators on semiconductor wafers, such as silicon (Si) or gallium arsenide (GaAs) wafers. For example, in an article entitled xe2x80x9cAcoustic Bulk Wave Composite Resonatorsxe2x80x9d, Applied Physics Letters, Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layer of zinc oxide (ZnO) sputtered over a thin membrane of silicon (Si). Further, in an article entitled xe2x80x9cAn Air-Gap Type Piezoelectric Composite Thin Film Resonatorxe2x80x9d, I5 Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara, a bulk acoustic wave resonator having a bridge structure is disclosed.
FIG. 2 shows one example of a bulk acoustic wave resonator having a bridge structure. The structure comprises a membrane 130 deposited on a substrate 200. The resonator further comprises a bottom electrode 110 on the membrane, a piezoelectric layer 100, and a top electrode 120. A gap 210 is created between the membrane and the substrate by etching away some of the substrate from the top side. The gap serves as an acoustic isolator, essentially isolating the vibrating resonator structure from the substrate.
In the following, certain types of BAW resonators are first described. Bulk acoustic wave resonators are typically fabricated on silicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. One further ceramic substrate type used is alumina. The BAW devices are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition. BAW devices utilize a piezoelectric thin film layer for generating the acoustic bulk waves. The resonance frequencies of typical BAW devices range from 0.5 GHz to 5 GHz, depending on the size and materials of the device. BAW resonators exhibit the typical series and parallel resonances of crystal resonators. The resonance frequencies are determined mainly by the material of the resonator and the dimensions of the layers of the resonator.
A typical BAW resonator consists of three basic elements:
an acoustically active piezoelectric layer,
electrodes on opposite sides of the piezoelectric layer, and
acoustical isolation from the substrate.
The piezoelectric layer may be for example, ZnO, AlN, ZnS or any other piezo-electric material that can be fabricated as a thin film. As a further example, also ferroelectric ceramics can be used as the piezoelectric material. For example, PbTiO3 and Pb(ZrxTi1-x)O3 and other members of the so called lead lanthanum zirconate titanate family can be used.
The material used to form the electrode layers is an electrically conductive material. The electrodes may be comprised of for example any suitable metal, such as tungsten (W), aluminum (Al), copper (Cu), molybdenum (Mo), nickel (Ni), titanium (Ti), niobium (Nb), silver (Ag), gold (Au), and tantalum (Ta). The substrate is typically composed of for example Si, SiO2, GaAs, glass, or ceramic materials.
The acoustical isolation can be produced with for example the following techniques:
with a substrate via-hole,
with a micromechanical bridge structure, or
with an acoustic mirror structure.
In the via-hole and bridge structures, the acoustically reflecting surfaces are the air interfaces below and above the devices. The bridge structure is typically manufactured using a sacrificial layer, which is etched away to produce a free-standing structure. Use of a sacrificial layer makes it possible to use a wide variety of substrate materials, since the substrate does not need to be modified very much, as in the via-hole structure. A bridge structure can also be produced using an etch pit structure, in which case a pit has to be etched in the substrate or the material layer below the BAW resonator in order to produce the free standing bridge structure.
FIG. 3 illustrates one example of various ways of producing a bridge structure. Before the deposition of other layers of the BAW structure, a sacrificial layer 135 is deposited and patterned first. The rest of the BAW structure is deposited and patterned partly on top of the sacrificial layer 135. After the rest of the BAW structure is completed, the sacrificial layer 135 is etched away. FIG. 3 shows also the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. The sacrificial layer can be realized using for example ceramic, metallic or polymeric material.
In the via-hole structure, the resonator is acoustically isolated from the substrate by etching away the substrate from under a major portion of the BAW resonator structure. FIG. 4 shows a via-hole structure of a BAW resonator. FIG. 4 shows the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. A via-hole 211 has been etched through the whole substrate. Due to the etching required, via-hole structures are commonly realized only with Si or GaAs substrates.
A further way to isolate a BAW resonator from the substrate is by using an acoustical mirror structure. The acoustical mirror structure performs the isolation by reflecting the acoustic wave back to the resonator structure. An acoustical mirror typically comprises several layers having a thickness of one quarter wavelength at the center frequency, alternating layers having differing acoustical impedances. The number of layers in an acoustic mirror is typically ranging from three to nine. The ratio of acoustic impedance of two consecutive layers should be large in order to present as low acoustic impedance as possible to the BAW resonator, instead of the relatively high impedance of the substrate material. In the case of a piezoelectric layer that is one quarter of the wavelength thick, the mirror layers are chosen so that as high acoustic impedance as possible is presented to the resonator. This is disclosed in U.S. Pat. No. 5,373,268. The material of the high impedance layers can be for example gold (Au), molybdenum (Mo), or tungsten (W), and the material of the low impedance layers can be for example silicon (Si), polysilicon (poly-Si), silicon dioxide (SiO2), aluminum (Al), or a polymer. Since in structures utilizing an acoustical mirror structure, the resonator is isolated from the substrate and the substrate is not modified very much, a wide variety of materials can be used as a substrate. The polymer layer may be comprised of any polymer material having a low loss characteristic and a low acoustic impedance. Preferably, the polymer material is such that it can withstand temperatures of at least 350xc2x0 C., since relatively high temperatures may be needed during deposition of other layers of the acoustical mirror structure and other structures. The polymer layer may be comprised of, by example, polyimide, cyclotene, a carbon-based material, a silicon-based material or any other suitable material.
FIG. 5 shows an example of a BAW resonator on top of an acoustical mirror structure. FIG. 5 shows the substrate 200, the bottom electrode 110, the piezo-electric layer 100, and the top electrode 120. The acoustical mirror structure 150 comprises in this example three layers 150a, 150b. Two of the layers 150a are formed of a first material, and the third layer 150b in between the two layers is formed from a second material. The first and second materials have different acoustical impedances as described previously. The order of the materials can be varied. For example, the material with a high acoustical impedance can be in the middle and the material with a low acoustical impedance on both sides of the middle material, or vice versa. The bottom electrode may also be used as one layer of the acoustical mirror.
FIG. 6 shows a further example of a BAW resonator structure. The BAW resonator illustrated in FIG. 6 is a stacked resonator structure having two piezoelectric layers 100. In addition to the bottom 110 and top 120 electrodes, a stacked structure requires a middle electrode 115, which is connected to ground potential. FIG. 6 further shows the membrane layer 130, the substrate 200 and the etch pit 210 isolating the structure from the substrate.
Radio frequency filters, for example, may be constructed using piezoelectric resonators. The resonance frequencies of SAW resonators and of thin film BAW resonators are such that it is advantageous to use them in filters, which are designed to operate at a certain frequency band at a frequency range from about 1 GHz to a few GHz. FIG. 7a presents an example of a ladder filter block 700 consisting of a resonator 701 connected in series and of a second resonator 702 connected in parallel. The series resonance frequency of the resonator 701 is typically at or close to the parallel frequency of the resonator 702, which is the center frequency of the ladder filter block 700. A ladder filter is typically constructed by connecting a certain number of ladder filter blocks 700 in series, for achieving the desired characteristics of the filter. FIG. 7b presents an example of a lattice filter structure 710, where resonators 701a and 701b are connected in series and resonators 702a and 702b are connected in parallel.
The manufacture of piezoelectric resonators involves a step of depositing a layer of piezoelectric material on a substrate. Using, for example, a suitable mask, it is possible to pattern the piezoelectric layer so that it covers only the areas of the piezoelectric resonators. As many piezoelectric materials are hygroscopic, usually a protective layer is used to cover the piezoelectric layer. Generally vias, which expose the bottom electrode, are manufactured to an unpatterned piezoelectric layer and to the protective layer. If the piezolayer is suitably patterned, it may be enough to make vias to the protective layer. The vias enable the connection of the bottom and top electrodes at desired places: conductive strips connected to the bottom electrodes become exposed, and the piezoelectric resonators can be easily connected to the rest of the electrical circuitry.
U.S. Pat. Nos. 5,231,327 and 5,404,628 describe a method for minimizing the number of vias in circuitries comprising BAW resonators: a BAW resonator in a circuitry can be replaced by a pair of BAW resonators having a shared electrode. This way the electrodes, using which the pair of BAW resonators is connected to the other circuitry, are on the same side of an unpatterned piezoelectric layer and the connection of the piezoelectric resonators to the other circuitry on the same substrate is possible even without making vias to the piezoelectric layer or patterning the piezoelectric layer.
In general, one aim in circuit design in to keep the number of resonators, as well as the number of other components, as small as possible. This way the manufacture costs can usually be kept as low as possible. Furthermore, typically a larger number of components in a circuitry, for example in a filter, causes larger attenuation. In filters where it is acceptable to use a patterned piezoelectric layer in the filter structure (in other words, the manufacture of which filters involves a patterning of the piezoelectric layer), it seems inefficient to use, for example, resonator pairs instead of single resonators.
As desirable as the use of SAW or BAW resonators would be in radio frequency filters and in other circuitry, unfortunately neither SAW resonators nor BAW resonators can withstand high power levels. The power handling capacity of radio frequency filters is crucial for, for example, transmitters of mobile communication devices. When too high a power level is exerted on a circuitry, the BAW resonators typically break so fast that it is difficult to analyze the cause of the breakdown. Often such a BAW resonator is so badly destroyed in the breakdown that only the bottom electrode resides on the substrate after the breakdown.
A further problem is that at power levels, which do not damage piezoelectric resonators of a filter, the heat load may cause the temperature of piezoelectric resonators to increase. This, in turn, typically causes the resonance frequencies of the piezoelectric resonators to change and the characteristics of a filter to change. For example, increase in temperature may cause the band pass of a filter to shift.
An object of the invention is to present a filter structure, which comprises piezoelectric resonators and which has good power handling capacity and good electric response. A second object of the invention is to present a transmitting arrangement which has good power handling capacity. A further object is to present a method for designing such electrical circuitry, which comprises piezoelectric resonators and has good power handling capacity and good electric response.
Objects of the invention are achieved by using in a filter structure at least one group of piezoelectric resonators, which all have a certain resonator frequency, which are connected in series with each other and which are connected to an input conductor of the filter structure.
The invention relates to a filter structure having a certain impedance level and comprising a first piezoelectric resonator, whose resonance frequency is a first resonance frequency and which is connected to an input conductor of said filter structure, said filter structure further comprising, in order to increase the power handling capacity of the filter structure, a chain of piezoelectric resonators, said chain comprising at least two piezoelectric resonators, connected in series with the first piezoelectric resonator and forming together with the first piezoelectric resonator a plurality of piezoelectric resonators connected in series, and wherein
each piezoelectric resonator belonging to said chain of piezoelectric resonators has a resonance frequency substantially equal to the first resonance frequency,
said plurality of piezoelectric resonators is connected to the rest of the filter structure only through the first piezoelectric resonator at one end of said plurality of piezoelectric resonators connected in series and through a second piezoelectric resonator, which is at the other end of said plurality of piezoelectric resonators connected in series, and
impedance of said plurality of piezoelectric resonators connected in series is arranged to match the impedance level of the filter structure.
The invention relates also to a filter structure having a certain impedance level and comprising a first surface acoustic wave resonator, whose resonance frequency is a first resonance frequency and which is connected to an input conductor of said filter structure, said filter structure further comprising, in order to increase the power handling capacity of the filter structure, a second surface acoustic wave resonator, a first electrode of said first surface acoustic wave resonator being connected to a first electrode of said second surface acoustic wave resonator, and said second surface acoustic wave resonator forming together with the first surface acoustic wave resonator a plurality of piezoelectric resonators connected in series, and wherein
each piezoelectric resonator belonging to said plurality of piezoelectric resonators has a resonance frequency substantially equal to the first resonance frequency,
said plurality of piezoelectric resonators is connected to the rest of the filter structure only through a second electrode of the first surface acoustic wave resonator and through a second electrode of the second surface acoustic wave resonator, and
impedance of said plurality of piezoelectric resonators connected in series is arranged to match the impedance level of the filter structure.
The invention further relates to a filter structure having a certain impedance level and comprising a first bulk acoustic wave resonator, whose resonance frequency is a first resonance frequency and which is connected to an input conductor of said filter structure, said filter structure further comprising, in order to increase the power handling capacity of the filter structure, a second bulk acoustic wave resonator, a first electrode of said first bulk acoustic wave resonator being connected to a first electrode of said second bulk acoustic wave resonator, and said second bulk acoustic wave resonator forming together with the first bulk acoustic wave resonator a plurality of piezoelectric resonators connected in series, and wherein
each piezoelectric resonator belonging to said plurality of piezoelectric resonators has a resonance frequency substantially equal to the first resonance frequency,
said plurality of piezoelectric resonators is connected to the rest of the filter structure only through a second electrode of the first bulk acoustic wave resonator and through a second electrode of the second bulk acoustic wave resonator,
the first bulk acoustic wave resonator and the second bulk acoustic wave resonator are not formed using a single unpatterned piezoelectric layer, and
impedance of said plurality of piezoelectric resonators connected in series is arranged to match the impedance level of the filter structure.
The invention further relates to a filter structure comprising
a first filter branch for filtering a first signal, said first filter branch having a first input conductor, a first output conductor and a plurality of piezoelectric resonators connected in series, said plurality having at least two piezoelectric resonators, and
a second filter branch for filtering a second signal, said second filter branch having a second input conductor and a second output conductor and said first output conductor being connected to said second input conductor,
wherein
each piezoelectric resonator belonging to said plurality of piezoelectric resonators has a substantially same resonance frequency,
said plurality of piezoelectric resonators is connected to the rest of the first filter branch only through a first piezoelectric resonator at one end of said plurality of piezoelectric resonators connected in series, said first piezoelectric resonator being connected to the first input conductor, and through a second piezoelectric resonator, which is at the other end of said plurality of piezoelectric resonators connected in series, and
impedance of said plurality of piezoelectric resonators connected in series is arranged to match the impedance level of the first filter branch.
The invention relates also to an arrangement for transmitting and receiving radio frequency signal, said arrangement comprising
first amplification means for amplifying a first signal,
second amplification means for amplifying a second signal, and
a filter structure comprising
a first filter branch for filtering the first signal, said first filter branch having a first input conductor, a first output conductor and a plurality of piezoelectric resonators connected in series, said plurality having at least two piezoelectric resonators, and
a second filter branch for filtering the second signal, and said second filter branch having a second input conductor and a second output conductor,
wherein
said first output conductor is connected to said second input conductor, said first input conductor is coupled to an output of the first amplification means and said second output conductor is coupled to an input of the second amplification means,
each piezoelectric resonator belonging to said plurality of piezoelectric resonators has a substantially same resonance frequency,
said plurality of piezoelectric resonators is connected to the rest of the first filter branch only through a first piezoelectric resonator at one end of said plurality of piezoelectric resonators connected in series, said first piezoelectric resonator being connected to the first input conductor, and through a second piezoelectric resonator, which is at the other end said plurality of piezoelectric resonators connected in series, and
impedance of said plurality of piezoelectric resonators connected in series is arranged to match the impedance level of the first filter branch.
The invention further relates to an arrangement for transmitting radio frequency signal, comprising
amplification means for amplifying a radio frequency signal, and
a filter structure for filtering the amplified radio frequency signal, said filter structure comprising a plurality of piezoelectric resonators connected in series, said plurality having at least two piezoelectric resonators,
wherein
each piezoelectric resonator belonging to said plurality of piezoelectric resonators has a substantially same resonance frequency,
said plurality of piezoelectric resonators is connected to the rest of the filter structure only through a first piezoelectric resonator at one end of said plurality of piezoelectric resonators connected in series, said first piezoelectric resonator being connected to an input conductor of said filter structure, and through a second piezoelectric resonator, which is at the other end of said plurality of piezoelectric resonators connected in series, and
impedance of said plurality of piezoelectric resonators connected in series is arranged to match the impedance level of the filter structure.
A method according to the invention is a method for designing a filter comprising the following steps of:
specifying a filter construction, which comprises piezoelectric resonators, said filter construction achieving a target frequency response,
replacing in the filter construction an original piezoelectric resonator having a first resonance frequency with a plurality of piezoelectric resonators, each of which has a resonance frequency substantially equal to the first resonance frequency, which are connected in series, and the impedance of the plurality of resonators being connected in series being the same as the impedance of the original piezoelectric resonator, and
selecting the number of piezoelectric resonators in said plurality of piezoelectric resonators.
The accompanying dependent claims describe some preferred embodiments of the invention.
The breakdown of a piezoelectric BAW resonator may be caused by an electrical breakdown caused by a high voltage and/or by the amplitude of the piezoelectrically excited mechanical vibrations of the resonator. The electrical breakdown and amplitude of mechanical vibrations that should not be exceeded suggest that there is an upper limit for the voltage which can be exerted over a piezoelectric resonator without damaging the resonator. When instead of a single piezoelectric resonator a number of piezoelectric resonators connected in series are used, the voltage exerted over each piezoelectric resonator is decreased. The upper limit for the voltage is thus reached at higher power than when a single piezoelectric resonator is used.
In piezoelectric SAW resonators, the voltage exerted over a SAW resonator is a significant factor causing damage to SAW resonators. Therefore a filter structure, where there are a number of piezoelectric SAW resonators connected in series, has a better power handling capacity than a filter structure having a single corresponding SAW resonator.
As the impedance of a number of piezoelectric resonators connected in series typically needs to be match with the resonance of the filter circuitry, the capacitance of the individual piezoelectric resonators belonging to the series of piezoelectric resonators are larger than the capacitance of a single piezoelectric resonator. The capacitance of a piezoelectric BAW resonator, for example, is proportional to the area of a resonator. Therefore the impedance matching causes the total area of the piezoelectric BAW resonators connected in series to be considerably larger than the area of a single piezoelectric BAW resonator. The impedance of a SAW resonator depends mostly on the number of fingers and on the length of the fingers. Similarly, the area of a plurality of piezoelectric SAW resonators connected in series and having a suitable impedance is typically larger than that of a single SAW resonator. The increased area enhances the heat transfer from the piezoelectric resonators to the substrate, and the temperature of a number of piezoelectric resonators connected in series increases typically less than the temperature of a single piezoelectric resonator in same circumstances. This helps to maintain the characteristics of a filter or other circuitry unchanged. Furthermore, it is possible that the breakdown of piezoelectric resonators isxe2x80x94at least partlyxe2x80x94caused by too high a temperature. The increased heat transfer capacity may thus directly enhance also the power handling capacity of a circuitry comprising piezoelectric resonators.
The attenuation of a filter comprising a number of piezoelectric BAW resonators connected in series is typically not significantly increased when compared to a filter comprising a corresponding single piezoelectric BAW resonator. This is due to the fact that as the area of a piezoelectric BAW resonator increases, the effective piezoelectric coupling coefficient of a piezoelectric BAW resonator increases. This is mostly due to the decreasing effect of the stray capacitance. The effective piezoelectric coupling coefficient is a measure of how efficiently a voltage exerted on a piezoelectric resonator is turned into mechanical vibrations in the piezoelectric resonator, and it is strongly dependent on the frequency of the exerted voltage. Thus, even though each piezoelectric BAW resonator increases the attenuation of the filter, the band pass of a filter can be designed to be a bit wider (the increased effective coupling coefficient enables this) and the increase in attenuation can be at least partly compensated.
When power handling capacity of a filter causes concern, the use of a number of piezoelectric resonators connected in series is thus advantageous in filter structures, irrespective of the manufacture of the piezoelectric resonators involving patterning of a piezoelectric layer or not. As the number and area of the piezoelectric resonators increases, the area of the filter structure typically increases. The advantages of using a number of piezoelectric resonators connected in series are, however, in many situations so significant that the increase in the filter area is acceptable. The area required to fit in a filter comprising thin-film piezoelectric resonators connected in series is considerably smaller than, for example, an area required to fit in a ceramic filter. Ceramic filters are nowadays used, for example, in cellular telephones.